fix: add Field alias for SetPartitionStatisticsUpdate (#3035)

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# Rationale for this change

Add `Field(alias="partition-statistics")` to
`SetPartitionStatisticsUpdate.partition_statistics` to ensure correct
serialization/deserialization with the hyphenated key format.

## Problem
The `partition_statistics` field was missing an explicit `Field` alias,
which means:
- When serializing with `model_dump(by_alias=True)`, it would use the
Python attribute name `partition_statistics` (with underscore) instead
of the Iceberg specification format `partition-statistics` (with hyphen)
- This causes incompatibility with the Iceberg table metadata format
specification

## Solution
Added `Field(alias="partition-statistics")` to ensure:
- Proper serialization to JSON/dict using hyphenated key names that
comply with Iceberg spec
- Correct deserialization when parsing external metadata with hyphenated
keys
- Consistency with other similar fields in the codebase (e.g.,
`snapshot_ids` with alias `snapshot-ids`)

## Are these changes tested?

Yes. Added verification in `test_set_partition_statistics_update()` to
validate that:
1. The update object serializes to JSON with the correct
`"partition-statistics"` key
2. The key is present in the serialized output

## Are there any user-facing changes?

No. This is an internal fix to ensure metadata serialization format
compliance. The change is transparent to users and improves
interoperability with the Iceberg specification.

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Author: shiwk

pyiceberg/table/update/__init__.py

@@ -212,7 +212,7 @@ class RemoveSchemasUpdate(IcebergBaseModel):
 
 class SetPartitionStatisticsUpdate(IcebergBaseModel):
     action: Literal["set-partition-statistics"] = Field(default="set-partition-statistics")
-    partition_statistics: PartitionStatisticsFile
+    partition_statistics: PartitionStatisticsFile = Field(alias="partition-statistics")
 
 
 class RemovePartitionStatisticsUpdate(IcebergBaseModel):

tests/table/test_init.py

@@ -1605,6 +1605,10 @@ def test_set_partition_statistics_update(table_v2_with_statistics: Table) -> Non
         partition_statistics=partition_statistics_file,
     )
 
+    # Verify that serialization uses 'partition-statistics' alias
+    dumped_with_alias = update.model_dump(by_alias=True)
+    assert "partition-statistics" in dumped_with_alias
+
     new_metadata = update_table_metadata(
         table_v2_with_statistics.metadata,
         (update,),